The Skinny on Acne

Hair Follicles
A hair grows inside a hair follicle. This is basically a “bag” with a canal that opens to the skin at the skin pore. The follicle is lined by specialized cells called keratinocytes. The skin and the hair are lubricated by oil (sebum) that is produced by oil glands (sebaceous glands) that surround the follicle and open into the follicular canal. See the figure below.
normal-psu

Whiteheads
The skin pore is only slightly larger than the hair, so there is only a small amount of space for the oil to escape. The pores can easily get plugged and cause the accumulation of oil and dead cells inside the hair follicle. This causes the follicle to swell and enlarge. Such an enlarged follicle that is still closed can be seen as a small bump on the skin and is referred to as a Whitehead or a Closed Comedo (see figure below).
closed-comedo

Blackheads
When the swelling reaches the surface of the skin, the trapped oil (sebum) is exposed to air which turns it dark.  This stage is called a Blackhead or Open Comedo. Thus this has nothing to do with dirt. Rather, the discoloration is a chemical reaction between the sebum and air. This process has very little to do with skin hygiene and it cannot be improved by washing.
open-comedo

Lesions
The accumulated mixture of oil (sebum) and dead cells is a good environment for the growth of bacteria. Many different kinds of bacteria normally live on the skin. One of the prominent species is Propionibacterium acnes. Once P. acnes bacteria get a foot-hold in the plugged follicle an infection is started. The bacteria attract white blood cells that engulf the bacteria and destroy them. If there are a lot of bacteria, the white blood cells “eat” so many bacteria that they themselves die in the process. This results in inflammation and exhibits itself as redness and tenderness. In extreme cases the follicles fill with puss, i.e. a mix of dead blood cells, shed skin cells, oil, and bacteria.
When the walls of the plugged follicles rupture under the pressure, everything spills into the surrounding skin tissue and forms a lesion.

Acne Lesions

Dermatologists usually distinguish four classes of Acne lesions, based on their severity:
  • Papules
    This is the mildest stage and is characterized by a mild inflammation and pinkish or red and tender bumps on the skin.
  • Pustules
    This is a more advanced stage and consists of papules that are red with white or yellow tips containing puss.
  • Nodules
    These are larger lesions that reach deep under the skin and can be quite painful.
  • Cysts
    These are the most severe stages of acne and consist of painful lesions that are so deep and big that they can result in permanent scarring.


[The figures on this page are from a public domain publication of the National Institute of Arthritis and Muscoskeletal and Skin Diseases (NIAMS). ]